Tuesday, November 17, 2009

Dark craters on a bright ejecta blanket

Dark materials excavated by later small impacts show up clearly on the bright ejecta of a small lunar crater (800 meters in diameter) to the west. Image width is 640 meters [NASA/GSFC/Arizona State University].

Samuel LawrenceLROC News System

Lunar geologists are interested in all aspects of craters, including their formation, structure, and the materials that they excavate. In this view of the ejecta blanket of a small fresh crater in the farside highlands north of Jules Verne Z, we can see where small, presumably secondary craters (that is, impact craters formed by the ejecta that expelled from a larger crater) have punched through the thin ejecta blanket of this fresh crater and excavated the darker, more mature materials beneath.

In this NAC view, it we can make a convincing argument that the fresh crater in the northwest part of the full NAC frame just outside of the field of view excavated fresh, immature materials, creating the bright ejecta blanket. Small secondary craters from other subsequent impacts (like the ones you see here) then excavated the mature materials beneath. By investigating these darker materials, we will gain insights into the composition and nature of the lunar crustal materials that are currently covered up by the bright ejecta blanket.

This is a (relatively) small-scale example; lunar scientists have used this tactic in other areas of the Moon to identify ancient buried mare deposits, or cryptomare (Read more about cryptomare!). Sampling cryptomare is one of the highest priorities of future human lunar exploration in order to tell lunar scientists how mare volcanism changed over billions of years of lunar geologic history and thus key insights into the origin and evolution of terrestrial planets. Several proposed Exploration sites are therefore locations where human explorers could directly explore and sample these ancient mare basalts.